Preheat oven to 425 degrees. Set rack in middle of oven.
Line muffin pans with paper liners and coat the top of the pans, between the cups, liberally with baking spray. Place the pans on parchment-lined cookie sheets (though it may seem like overkill, doing this greatly reduces the chances of the muffin bottoms over-browning, a danger with jumbo muffins since they need to bake substantially longer than smaller muffins).
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside. In the bowl of your mixer, using the paddle attachment, blend the oil, melted butter and sugar together. Quickly add in the eggs and vanilla extract and blend well. Remove bowl from the mixer and, using a rubber spatula, fold in the flour mixture. Or, if your mixer has a slow “stir” speed, use that. On low speed, blend in the buttermilk and sour cream. The batter should be thick.
If you prefer, you may stir the berries into the batter at this point, but doing so may turn the batter blue and the blueberries may burst. Another option is to fill the muffin cups about one third of the way full with batter, then scatter some berries into each cup. Then, cover those berries in each cup with another spoonful of batter, and then add another bunch of berries on top of that. Finally, top all of that off with a last little dab of batter. If using the optional lemon zest, stir together grated lemon zest and coarse sugar until evenly combined. Sprinkle coarse sugar generously over the top of each muffin.
Bake the muffins, if jumbo sized, for 15 minutes at 425 degrees, then turn the temperature down to 350 degrees and continue baking 12 to 16 more minutes. The muffin tops should be lightly golden, and a toothpick inserted deeply into the center of one of the middle muffins should come out mostly clean. The muffins should also spring back when lightly pressed with your finger. (If baking regular size muffins, they'll bake in half the time.).
Let them cool in their pans for about 5 minutes before you attempt to remove them. If the large muffin tops are a little stuck to the top of the pan, slide a knife blade gently underneath to loosen them first. Let them cool further on a rack. Delicious served warm or cold.